The present invention relates to an auxiliary power unit, and more particularly to a control system for an auxiliary power unit which utilizes a permanent magnet generator.
An Auxiliary Power Unit (APU) system is often utilized in conjunction with main propulsion engines to provide a secondary power sources. An APU system, typically in the form of an independent gas turbine engine, provides shaft power to drive an electric generator and interconnected electrical systems.
Some APU systems utilize a permanent magnet generator driven directly by the gas turbine engine. A directly driven generator minimizes the requirement for a relatively complicated gearbox, clutch interconnections, and result in lighter and more compact APU systems. APU speed excursions adversely affect the permanent magnet generator output By design, the APU during electrical load generation is constrained to relatively strict speed limits in order to meet the permanent magnet generator electrical output requirements.
The permanent magnet generator output capability is dependent on the gas turbine engine operation. Large transient load applications or load sheds can produce electrical demand changes that affect the level of APU generator output voltage. If the gas turbine engine control system does not respond to changes in the electrical load, the APU generator output voltage can deviate outside of specified limits and fail to provide sufficient power to meet electrical system requirements.
Accordingly, it is desirable to provide a control system for an APU system with a permanent magnet generator which maintains the generator output voltage within specified limits regardless of the load drawn by the vehicle.
The APU system according to the present invention provides an APU system which drives a permanent magnet generator, an APU controller and a fuel controller. The APU is preferably an independent gas turbine engine, separate from an associated main engine, such as a main vehicle propulsion engine.
As the current drawn from the generator increases due to the addition of applied loads to the generator, the output voltage will fall. The APU controller of the present invention controls the APU speed through communication with the fuel controller to maintain the generator voltage output within specified limits in response to an engine speed set point signal, a voltage set point signal and a voltage output signal.
The present invention therefore provides a control system for an APU system with a permanent magnet generator which maintains the generator output voltage within specified limits regardless of the load current being drawn by the aircraft.